The invention relates to an inorganic, heat resistant material for devices for forming structural elements of optical glass. After the forming proccess, the structural elments may be used without any further surface treatment, such as grinding or and/or polishing, because of the accuracy and quality of the formed surface.
The production of optical structural elements, from glass is a multistage process, requiring high accuracy. In addition to the preparation of the geometrical shape, for example of curved surfaces with certain predetermined radii and their centering, the surfaces must be free of surface defects, such as scratches, grooves, etc. The surfaces must be "finish polished". These processes are time and cost intensive.
In order to shorten the grinding or milling process of optical elements (lenses, prisms, mirrors, panes), pressed glass blanks are prepared. The dimensions of the blanks correspond to those of the finished optical part, with corresponding overmeasure. The blanks are pressed from glass at a temperature such that the glass becomes plastic with a high viscosity. Heat resistant metal alloys free of forging scales are used as mold components.
The blanks prepared in this manner have a more or less dull surface, originating in the reaction of the glass with the die material and the powdered parting agents used during the process between the surface of the glass and other contact surfaces. These blanks must then be ground and polished in the conventional manner.
By providing the molds with an appropriately fine surface, it is possible to press lenses with a surface quality permitting their use in the illuminating beam path of optical insturments. However, their surfaces still have enough defects in the microrange ("orange peel effect"), so that lenses produced in this manner are not suitable for use in an optical imaging beam path.
As an improvement, the use of glass carbon on the mold surfaces is proposed in U.S. Pat. Nos. 3,900,328, 3,833,347 and 3,844,755. Optical surfaces produced by dies with mold surfaces of glass carbon are actually of a significantly higher quality. However, glass carbon oxidizes and has further detrimental properties.
A further improvement is proposed in DE-OS No. 2 639 259. The material forming the mold surfaces here consists of silicon carbide (SiC) or silicon nitride (Si.sub.3 N.sub.4). However, such mateials also have significant disadvantages. Both SiC and Si.sub.3 N.sub.4 are sensitive to oxygen and water vapor at elevated temperatures. The materials may decompose according to the following reaction equation: EQU SiC+20.sub.2 .fwdarw.SiO.sub.2 +CO.sub.2
and silicon carbide and water convert to silicon dioxide and methane according to: EQU SiC+2H.sub.2 O.fwdarw.SiO.sub.2 +CH.sub.4.
Initially, these reactions occur superficially and lead to a coating of the aforementioned materials by a protective SiO.sub.2 layer. In the case of extended exposure to elevated temperatures, the reaction front penetrates into the volume of the material, whereby its volume is increased and the SiC or Si.sub.3 N.sub.4, respectively, is decomposed and leaks out in the form of dust.
In addition to the reaction of atmospheric oxygen and water, the material of the mold surface must withstand the attack of the molten glass. It is generally true that the resistance of a molding tool is an inverse function of the chemical reaction gradient between the softened glass and the material of the mold. Reactions between softened glasses or glass melts and the surfaces of molding tools may occur already upon simple contact. Such reactions are designated contact reactions. According to their contact reactions, the following three groups of materials may be distinguished:
1. Acid group:
This includes SiO.sub.2 materials, but also silicon carbide and silicon nitride.
2. Basic group:
The group comprises CaO and/or MgO ceramics.
3. Group indifferent to contact:
Carbon and glazing carbon are included.
It is almost impossible to bring materials of the acid group into contact with materials of the basic group without incurring destruction by contact reactions. Glass melts, in particular those to be used for optical purposes, differ greatly in their chemical compositions, i.e., they may be both acidic or basic in nature.